Is striving after self-improvement inherently valuable, and if so, how? Otherwise, is it merely a means to a mundane end such as money, prestige, or such? If the latter is the case, is there some reason not to be content with only moderate success and/or exertion and how would this not lead to a slippery-slope of laziness?

I just responded to two other questions that are really related to this one, so please have a look at my replies to those others.

But to focus briefly on your question, I would say that it really depends upon what you mean by "self-improvement." This can come in loads of forms, of course, and some of them look pretty trivial to me. Consider the following forms of "self-improvement":

Getting richerCosmetic surgeryColoring one's hairGetting a tattooLearning how to use a new computer programLifting weights to get more "buff"

I don't mean to suggest that none of these can bring real improvement to a life--but I do tend to doubt that such improvements are really the most significant kinds.

But if one bcomes dedicated to such self-improvements as, for example, becoming better educated, or becoming more effectively involved in one's community, or becoming a kinder or more understanding person, then I really do think that the pursuit of self-improvement can itself be valuable, because such pursuits also have the effect of making one's life more meaningful (to oneself and also to others). The guiding principle, however, must be "moderation in all things." Too much dedication to "self-improvement" can actually end up being (and looking like) a selfish life, and so whatever gains in "self-improvement" one might accomplish would seem in such excessive cases to be offset by the degree to which the excessive focus on oneself leads to inadequate concern for others. But even attention to getting this balance right seems to me to be a very significant and valuable form of self-improvement!

On this issue, I would also recommend that you have a look at Aristotle's Nicomachean Ethics, and to give some thought to what sorts of values one is looking to improve upon in one's life--and whether or not those values are really worth pursuing and improving (i.e. whether they are intrinsic values, and important ones, or whether they are really only instrumental values, like wealth). But I discussed these in my other answers.